Cables for heavy-duty applications and in particular for mobile installations, such as mobile harbour cranes, ship-to-shore container cranes, ship un-loaders, spreaders, mining and tunneling equipment, and windmill and windfarm are specifically designed to withstand harsh environment conditions and high mechanical stresses, such as tensile forces and torques. Within the present description, we will in general refer to heavy-duty cables, when referring to cables for heavy-duty applications and in particular, but not exclusively, for mobile installations.
In some applications, such as in heavy-duty applications, transfer of the cable to the equipment reels and forced guidance during the winding and unwinding phases may give rise to undesired torsions that can vary along the cable length. Although care is normally recommended in handling and installation of the cable in the mobile equipments, such as a direct transfer of the cable from the original drum to the cable reel while avoiding changes of direction or inversions of the original direction of winding, working conditions may induce relatively large and abrupt torques thereof. In addition, other systems for cable movement, such as guidance devices, pulley systems and tender systems, may involve torsions of the cable during operation, in particular if applications require high-speed operation and/or multiple cable deflection in the cable payout.
Optical sensors useful for measuring and/or monitoring mechanical stresses in an electric cable are known.
WO 2010/136062 describes an electric cable comprising a strain sensor longitudinally extending along the cable and including a strain optical fibre arranged within a bending neutral region surrounding and including a bending neutral longitudinal axis of the electric cable, and at least two longitudinal structural elements, at least one of the at least two longitudinal structural elements being a core comprising an electrical conductor, wherein the strain sensor is embedded in a strain-transferring filler mechanically coupling at least one of the at least two longitudinal structural elements with the strain sensor. With the disclosed cable construction, the strain experienced by the at least one of the at least two longitudinal structural elements is transferred to the strain sensor at least in a strained condition.
WO 2011/032587 relates to a method for monitoring the torsion of a cable comprising the steps of providing a cable with at least one identification tag, preferably an RFID tag, arranged in a tag angular position in a cross-sectional plane taken transverse to the longitudinal direction and detecting the tag electromagnetic signal. The cable is provided with a plurality of identification tags, each tag being arranged in a respective tag angular position.
J. Burgmeier et al. in “Fiber optic sensor system for stress monitoring in power cables”, published in 2009 Conference on Lasers and Electro-Optics (CLEO), describe a fibre optic sensor system for monitoring stress factors such as temperature, squeezing, bending and torsion in power cables using a short pulse and broadband light source. Monitoring bending and torsion is performed via fibre Bragg gratings (FBGs) and uses a broadband source, achieved by supercontinuum generation. To use standard single mode fibres, which are frequently used within power cables for data transmission, the FBG is written into the fibre by point by point femtosecond laser inscription. Bending the fibre which is integrated into the power cable results in a change of the grating period of the FBG and thus different wavelengths from the broadband source will be reflected and easily monitored by a compact spectrometer.
Radiation loss occurs when a single mode fibre is bent. Polarisation-sensitive optical time domain reflectometry (P-OTDR) was proposed as a tool to measure the birefringence of single-mode optical fibres. P-OTDR provides the evolution of the state of polarisation (SOP) of the Rayleigh backscattering field, whereas information about the birefringence from the measured SOPs is derived by data modelling and analysis.
Polarization sensitive reflectometers are a special class of optical reflectometers that aims at measuring the state of polarization (SOP) of the optical field backscattered by an optical fibre, due to Rayleigh scattering, as a function of the position along the fibre where the scattering takes place. In general, the optical fibre under test is probed with a known, polarization-controlled, probe optical signal (e.g. a pulse or a frequency modulated signal), while the backscattered optical field is measured as a function of time with a polarization sensitive receiver. Owing to the knowledge of the probe signal and of the propagation speed of light in the specific fibre, it is then possible to convert the time variations in a longitudinal map of the local properties of the fibre under test.
A review of theory and applications of polarisation-sensitive optical time-domain reflectometry (P-OTDR), in particular related to polarisation mode dispersion (PMD) in single-mode fibres, is given is “Spatially Resolved PMD Measurements” by A. Galtarossa and L. Palmieri, published in Journal of Lightwave Technology, col. 22 (2004), pages 1103-1115.
A. Galtarossa et al., “Reflectometric measurement of birefringence rotation in single-mode fibers”, Optics Letters, vol. 33 (2008), pages 2284-2286, disclose a reflectometric technique for the measurement of orientation and modulus of the linear birefringence vector in single-mode optical fibres. The technique provides information also on circular birefringence, although this component, if present, appears as a rotation of the linear birefringence. Deterministic rotations may be caused by either a twist or a spin applied to the fibre.
A. Galtarossa et al., “Spin-profile characterization in randomly birefringent spun fibers by means of frequency-domain reflectometry”, Optics Letters, vol. 34 (2009), pages 1078-1080 show that the angle of rotation of birefringence, and hence the spin profile, of an optical fibre can be measured by polarisation-sensitive optical frequency-domain reflectometry (P-OFDR). The P-OFDR technique was applied to a fibre sample of a few tens of meters long.
The Applicant has tackled the problem of monitoring torsion in a cable in use and of providing a reliable measurement of the actual deployment of the cable, which can be performed, for example periodically, throughout the lifetime of the cable.
The Applicant has observed that a solution as that described in WO 2011/032587 provides information about the local rotational state of the cable, in particular on the longitudinal portion of the cable that has passed through the reading device capable of transmitting interrogation electromagnetic signals and of receiving the tag electromagnetic signal transmitted by the tag(s) placed across the cable portion detected by the reading device.
In some applications, such as in cables for heavy-duty applications, it is desirable to determine the torsion distributed along the cable length. In particular, it can be desirable to monitor the temporal evolution of the distributed torsional state along the cable, for example by comparing results from measurements taken at different times so that to adjust, if necessary, guiding rollers and reels. In some applications, evaluation of the torsional state along the cable can predict the residual lifetime of the cable.